Helium-cadmium laser and method for optimum operation

ABSTRACT

Means for introducing cadmium vapor into a helium laser tube and for maintaining optimum vapor pressure are disclosed in the form of a steel jacketed hollow cadmium cylinder mounted coaxially within the laser tube between the anode and cathode, and surrounded by an evacuated bulb. Plasma discharge of the ionized helium gas passes through and heats the hollow cadmium cylinder to produce cadmium vapor, thus eliminating any need for an auxiliary heater. A method of selecting and adjusting the plasma discharge current to optimum value is also disclosed.

imited States Patent 1 Rigden [111 3,748,595 1 July 24, 1973HELIUM-CADMIUM LASER AND METHOD FOR OPTIMUM OPERATION [75] Inventor:Jameson Dane Rigden, Westport,

Conn.

[73] Assignee: The Perkin-Elmer Corporation,

Norwalk, Conn.

[22] Filed: Mar. 6, 1970 [21] Appl. No.: 17,212

[52] U.S. Cl. 331/945, 330/43 [51] Int. Cl. H0ls 3/22 [58] Field ofSearch 331/945; 330/43 [56] References Cited UNITED STATES PATENTS3,621,460 11/1971 Willett 331/945 3,464,025 8/1969 Bell 331/945 PrimaryExaminer- Williams LfSikes Attorney -Edward R. Hyde, Jr.

[57] ABSTRACT 5 CIaiHiQ tEFaWiHg'Figures ZZ HIE/l6 PATENTEDJULEEMSISJameson Dane P d n BY 7 @M 62.6%

ATTORNEY HELIUM-CADMIUM LASER ANDMETIIOD FOR OPTIMUM OPERATIONBACKGROUND OF THE INVENTION The need for efficient low cost blue lightlaser sources has been recognized for some time. l-Ieretoforeultraviolet lasers have been developed employing argon or neon in ahighly excited state, requiring high input power and costly equipment.More recently heliumcadmium lasers have been announced by Bell TelephoneLaboratories (Electronic Design News July 1, 1969, p. 22) and by the RCACorporation (RCA Review Sept. 1969, pp. 422-428). In thisesheliumcadmium lasers it is necessary to produce and maintain a partialpressure of approximately two microns of cadmium vapor pressure in ahelium buffer which is maintained at a pressure of several torr. Theexcitation of the cadmium is produced by penning collisions betweenhelium metastables and ground state cadmium atoms.

The exact pressure of cadmium vapor is quite critical for stable lasingaction. In the prior art this cadmium vapor is pro-duced by applyingexternal heat to metal pellets contained in one or more evaporatorchambers connected with the helium laser tube. The critical vaporpressure required has been maintained in the prior art devices by meansof servo temperature control of the evaporator heater to a necessaryaccuracy of better than :lC. Experience has shown that this means ofvapor pressure control is difficult to achieve, requires expensivemechanisms and has an undesirably long time lag constant. In practicewith the prior art techniques too much cadmium vapor is often produced,with the consequent danger of coating sensitive optical surfaces such aswindows and mirrors with condensed metal vapor, and the dischargebecomes quite unstable, a condition known in the art as the bluemeanies.

OBJECTS OF THE INVENTION A principal object of the present invention isto overcome the problems and difficulties inherent in the prior art asdescribed above.

A more particular object is to provide a simplified, self-regulatinghelium-cadmium laser which does not require external heat regulatingmeans.

Another object of the invention is to provide heliumcadmium lasers whichare less expensive to manufacture, and more reliable and stable inoperation.

An additional object of the invention is to provide a method ofselecting the proper value of discharge current to achieve optimum laseroperation.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the features of construction,combinations of elements, and arrangements of parts which will beexemplified in the constructions hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawing, in which:

FIG. 1 is a overall front view of a preferred embodiment of a laseraccording to the invention, with the metal vapor generator portion shownin broken lines;

FIG. 2 is an enlarged cross-sectional view of the metal vapor generatorportion taken along the line 2-2 of FIG. 1;

FIG. 3 is an axial cross-sectional view taken along the line 3-3 of FIG.2; and

FIG. 4 is a cross-sectional view of an alternative embodiment of theinvention wherein the cadmium evaporation chamber is surrounded by andenclosed within a Dewar vacuum bulb.

DESCRIPTION OF THE INVENTION Referring now to FIG. 2 of the drawing itwill be seen that the laser of the invention comprises an elongatedlinear discharge tube 10-10' which may be formed of glass with enlargedanode and cathode chambers 11 and 12 near the opposite ends thereof.Chamber 11 contains a metallic anode 14, while chamber 12 contains acorresponding cathode l6. Cathode 16 is heated by a filamentary heaterwinding and energized through a pair of conductors 17 (FIG. IA) broughtout through a seal in the glass envelope of chamber 12. Positivepotential is applied to anode 14 via a conductor 18 sealingly passedthrough the cylindrical envelope of chamber 11. Planar windows 19 and 20are sealingly affixed to the opposite ends of discharge tube 10-10 atBrewsters angle, in the manner well known in the art. Aligned sphericalmirrors 21 and 22 are mounted outside the window 19 and 20 with theircentral axes in exact alignment with the longitudinal axis of dischargetube 10 to reflect light waves generated by the gaseousdischarge backinto tube 10 and along the longitudinal axis thereof to generate laseraction, all as well known in the art. One of the mirrors 21-22 isnormally semitransparent to allow passage of a portion of the laser beamgenerated within tube 10-10 for use outside the tube.

Mounted on and surrounding discharge tube 10-10 near the anode chamber 11 is a cylindrical evaporation chamber indicated generally at 24 inFIG. 1. Chamber 24 is hermetically sealed to the coaxially aligneddischarge tube portions 10 and 10 by annular seals 25 and 26 as shown inFIG. 1. Within chamber 24 between the inner end 27 of discharge tube 10and the inner end 28 of discharge tube extension 10' is mounted,coaxially with the longitudinal axis of discharge tube 10-10, a hollowcylindrical metallic member indicated generally at 29. The metallicmember 29 comprises an inner hollow cylinder 30 formed of cadmium,surrounded by an outer cylindrical jacket 31 formed of stainless steel.The steel jacket 31 is made somewhat longer than the hollow cadmiumcylinder 30, and the inner diameter of steel jacket 31 is slightlylarger than the outer diameter of ends 27 and 28 of discharge tubeportions 10 and 10' respectively, whereby the combined hollow metalliccylinders 30 and 31 are supported by, and coacially with, the dischargetube sections 10 and 10'. When thus assembled and sealed, the entirelaser tube structure is evacuated through a tit 32 on the annularlyextending sealed portion 34 of chamber 24, and then the evacuated tubeis back-filled with helium gas at a suitable pressure which may be ofthe order of one or more torr.

The dimensions of the cadmium cylinder 30 are chosen empirically tomatch the bore of the laser discharge tube 10. I have found that with atube bore of 2.7 mm and a total plasma length of 50 cm. a cadmiumcylinder 1 inch long and having an inside diameter of three-eighth inchworks fune to produce laser light at 4,416 A. A laser built according tothese specifications has been found to last quite stably'with a plasmacurrent ranging from 100 milliamperes to over 150 milliamperes, and toproduce 30 milliwatts of 4,416 A light at 135 milliamperes. I have foundit desirable, however, to insulate the cadmium evaporation chamber 24from air drafts to prevent the evaporation temperature from fluctuating.For this purpose I wrap a layer 35 (FIG. 1) of asbestos around theexterior of the chamber bulb 24. No doubt other heat insulatingmaterials may be employed in lieu of asbestos, or this insulation may beprovided by surrounding the evaporation chamber 24 with an integrallyformed evacuated Dewar bulb as shown in FIG. 4.

Referring now to FIG. 4 of the drawings, the cadmium evaporation chamber24 is constructed in substantially the same manner as describedhereinabove with reference to FIG. 2. In this embodiment, however, theexternal cylindrical wall 36 of the evaporation chamber 24 is surroundedby a larger diameter cylindrical wall 37 the ends of which are sealed towall 36 by annular seals 38 and 39, forming an enclosed and hermeticallyseealed substantially cylindrical space 40 between walls 36 and 37 andsurrounding the chamber 24. The inner surfaces of annular chamber 40 maybe silvered in the manner employed in the manufacture of vacuum bottles,and then a vacuum is drawn within chamber 40 through an external titseal 41.

With the structures hereinabove described the application of suitableoperating potential, which I have found to be of the order of 2,000volts to produce a constant discharge current of at least 100milliamperes, the plasma discharge passes through the hollow cylindricalcore of the cadmium cylinder 30 on its way to the cathode 16. Thecylinder'30 is heated by the plasma and consequently releases cadmiumvapor into the plasma. The process of vapor release, and the resultingvapor pressure of cadmium in the plasma, is partially self-regulatingand remains a constant for any given dis charge current. Thisself-regulation is believed to result from the fact that plasmacontaining cadmium vapor has a lower impedance than plasma withoutcadmium and hence, with a constant current supply, any increase incadmium vapor pressure reduces-the amount of heat dissipated into thecadmium cylinder, with a consequent automatic reduction in cadmium vaporrelease. This inherent self-regulation of cadmium vapor pressure in thestructure described has a shorter time constant than when an externalheater is used and produces a much more stablehelium-cadmium laser thanhas heretofore been obtainable with the more complicated and costlyexternal servomechanisms employed for regulation of an auxiliary heater.

A further advantage of the helium-cadmium laser structure disclosed isthat it provides a simple means for determining and controllingoperation of the laser at optimum cadmium vapor pressure. A smallmodulation of the discharge tube current produces (in general) amodulation on the laser light at 4,416 A. If the laser light modulationis in phase with the tube current modulation, then the cadmium vaporpressure is too high and the laser is not operating at maximumefficiency. In this case the value of the discharge current should bereduced to reduce the cadmium vapor release. On the other hand, if thelaser light modulation appears 180 degrees out of phase with the currentdischarge modulation, then the cadmium vapor pressure is too low and thedischarge current should be increased to achieve more cadmium vaporrelease. If desired, a servo control may be employed to adjust thedischarge tube current in response to automatic sensor's responsive tothe above described modulation characteristics. Once theoptimumdischarge current has been determined in this manner, the properconstant operating current may be recorded on the name plate of eachmanufactured laser so that the purchaser may always be enabled to obtainoptimum performance without further testing or experimentation.

While the example given herein is for a laser working at 4,4] 6 A, it isto be understood that the structures disclosed may be modified withinthe skill of the art to adapt the invention to lasers operable at otherfrequencies, as for example an ultraviolet laser at 3,250 A may beproduced also using a hollow cadmium cylinder installed in the plasmadischarge path as shown in the drawings. It is also to be understoodthat lasers utilizing other metal vapors may employ the self-heating andself-regulating features of the structure which comprises the heart ofmy invention.

It will thus be seen that the objects set forth above, among those madeapparent from the proceding description, are efficiently attained and,since certain changes may be made in the above construction withoutdeparting from th scope of the invention, it is in tended that allmatter contained in the above description or shown in the accompanyingdrawing shall be interpreted as illustrative and not in a limitingsense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention which,as a matter of language, might be said to fall therebetween.

Having described my invention, what I claim as new and desire to secureby Letters Patent is:

1. In an optical laser tube employing helium gas and a metallic vapor asthe active medium, means for introducing metal vapor into the gas plasmacomprising, a hollow cylindrical tube formed of the metal to bevaporized, a cylindrical steel jacket surrounding said hollow tube, andmeans for mounting said-steel jacketed hollow metal tube in coaxialrelation within said laser tube, whereby plasma generated by theapplication of electrical power to said laser tube passes through theaxial bore of said hollow metal tube imparting heat thereto andpartially vaporizing metal therefrom.

2. The combination of claim 1 wherein electrical power is applied tosaid laser tube from a constant current source whereby heating of themetallic cadmium produces cadmium vapor which combining with the heliumgas reduces the electrical impedance thereof, thereby reducing the-'heatdissipated by the constant current discharge to effectivelyself-regulate the production of cadmium vapor.

3. In a helium-cadmium optical laser tube, means for introducing apartial pressure of cadmium vapor into a helium buffer within the lasertube comprising, a hollow cadmium cylinder having an axial boretherethrough, and means supporting said hollow cadmium cylinder with itsaxial bore coaxial within said laser tube, whereby plasma of said heliumbuffer passes through the axial bore of said hollow cadmium cylinderthereby heating and vaporizing cadmium from said cylinder into saidhelium buffer, and a forced fitting stainless steel cover tubegencircling said hollow cadmium cylinder to prevent vaporization exceptfrom the internal axial bore of said cadmium cylinder.

4. In a helium-cadmium laser the method of selecting a critical value ofconstant discharge current for optimum laser operation which comprisesthe steps of A. applying modulation to the laser tube current to producea modulation on the laser light,

B. comparing the phase relation between the modulations of tube currentand the modulation of laser light,

C. increasing the tube current if the modulations are 180 out of phase,and

D. decreasing the tube current if the modulations are in phase.

5. A helium-cadmium laser comprising in combination,

A. an elongated laser tube having a straight line central longitudinalaxis,

1. a pair of transparent planar windows sealingly mounted at oppositeends of said laser tube, a. said windows mounted at Brewsters angle,

2. a coaxial anode sealingly mounted within an enlarged portion of saidlaser tube adjacent one end thereof,

3. a coaxial cathode sealingly mounted within an enlarged portion ofsaid laser tube adjacent the opposite end thereof,

4. an enlarged cylindrical chamber sealingly mounted to and surroundingsaid laser tube at a position between said anode and cathode, in closerproximity to said anode,

a. a hollow metallic cylinder of cadmium inserted in said laser tube andwithin said enlarged cylindrical chamber, coaxial with the longitudinalcentral axis of said laser tube,

b. a close fitting stainless steel cylindrical tube surrounding theouter surface of said hollow cadmium cylinder,

c. insulating means surrounding the outer surface of said enlargedcylindrical chamber,

5. helium gas under pressure within said lase tube and,

B. a pair of concave spherical mirrors positioned at opposite ends ofsaid laser tube in proximity to said planar windows and having thecenters of their spherical surfaces normal to the longitudinal centralaxis of said laser tube.

UNITED STATES PATENT OFFICE v CERTIFICATE OF CDRRECTION Patent No. 3 748595 Dated July 24 1973 lnve'n fl Jameson Dane Riqden It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column l,line l3, "thises" should be these; line 24, "pro-duced" shouldbe --produced; line 65, "a" (first occurrence) should be an. Column 2,line 12, after "now to" insert FIG. 1 and-; line 28, "window" should bewindowsline 56, "coacially should be -coaxially--. Column 3, line 1"fune" should be -fine;

I line 3, "last" should read lase line 16, "drawings" should be drawingline 24, "seealed", should read sealed Column 6, line 17, "lase" shouldread laser Signed and sealed this 8th day of January 1974.

(SEAL) Attest:

EDWARD M. FLETCHER,JR. RENE D. TEGTMEYER Attesting Officer ActingCommissioner of Patents I M PC4050 {10-69) uscoMM-Dc scanoo fi US.GOVERNMENT PRINTING OFFICE l9! 0-36-3$4.

UNITED STATES PATENT OFFICE I CERTIFICATE OF CORRECTION Patent No. 3,74,595 Dated July 24, 1973 Invent Jameson Dane Riqden It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column l,line l3, "thises" should be these-; line 24, "pro-duced" shouldbe produced; line 65, "a" (first occurrence) should be -an-. Column 2,line 12, after "now to" insert FIG. 1 and; line 28, "window" should bewindows--; line '56, "coacially" should be -coaxially--. Column 3, line1 "fune" should be -fine-;

. line 3, "last" should read lase line 16, "drawings" should be drawingline 24, "seealed" should read sealed Column 6, line 17, "lase shouldread laser Signed and sealed this 8th day of January 1974.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. RENE D. TEGTMEYER Attesting Officer ActingCommissioner of Patents I M PC3-1050 no-ss) uscoMM-Dc wan- 69 a U. 5GOVERNMENT HUNTING OFFICE I!" O-J6-38|.

2. The combination of claim 1 wherein electrical power is applied tosaid laser tube from a constant current source whereby heating of themetallic cadmium produces cadmium vapor which combining with the heliumgas reduces the electrical impedance thereof, thereby reducing the heatdissipated by the constant current discharge to effectivelyself-regulate the production of cadmium vapor.
 2. a coaxial anodesealingly mounted within an enlarged portion of said laser tube adjacentone end thereof,
 3. a coaxial cathode sealingly mounted within anenlarged portion of said laser tube adjacent the opposite end thereof,3. In a helium-cadmium optical laser tube, means for introducing apartial pressure of cadmium vapor into a helium buffer within the lasertube comprising, a hollow cadmium cylinder having an axial boretherethrough, and means supporting said hollow cadmium cylinder with itsaxial bore coaxial within said laser tube, whereby plasma of said heliumbuffer passes through the axial bore of said hollow cadmium cylinderthereby heating and vaporizing cadmium from said cylinder into saidhelium buffer, and a forced fitting stainless steel cover tubeencircling said hollow cadmium cylinder to prevent vaporization exceptfrom the internal axial bore of said cadmium cylinder.
 4. In ahelium-cadmium laser the method of selecting a critical value ofconstant discharge current for optimum laser operation which comprisesthe steps of A. applying modulation to the laser tube current to producea modulation on the laser light, B. comparing the phase relation betweenthe modulations of tube current and the modulation of laser light, C.increasing the tube current if the modulations are 180* out of phase,and D. decreasing the tube current if the modulations are in phase. 4.an enlarged cylindrical chamber sealingly mounted to and surroundingsaid laser tube at a position between said anode and cathode, in closerproximity to said anode, a. a hollow metallic cylindEr of cadmiuminserted in said laser tube and within said enlarged cylindricalchamber, coaxial with the longitudinal central axis of said laser tube,b. a close fitting stainless steel cylindrical tube surrounding theouter surface of said hollow cadmium cylinder, c. insulating meanssurrounding the outer surface of said enlarged cylindrical chamber, 5.helium gas under pressure within said lase tube and, B. a pair ofconcave spherical mirrors positioned at opposite ends of said laser tubein proximity to said planar windows and having the centers of theirspherical surfaces normal to the longitudinal central axis of said lasertube.
 5. A helium-cadmium laser comprising in combination, A. anelongated laser tube having a straight line central longitudinal axis,